Waste sludge may originate from a mechanical, biological or chemical process. Treated waste sludge often is a mixture of different types of waste sludges from these processes. Current efforts of sludge treatment are aimed at treating community or industrial wastewater sludge, industrial flour sludge and agricultural animal excrement sludge in such a way as to cause as few environmental problems as possible, and also, on the other hand, to utilize the sludge as efficiently as possible.
Currently, waste sludge is often treated by means of composting, which is a time-consuming process requiring periodic mechanical handling of the compost material to improve aeration. Lately it has become common practice to burn sludge, especially in the wood processing industry, which is not an efficient method due to the high moisture content of the sludge, but which makes it possible to get rid of undesirable waste.
For a long time it has been understood that it would be advantageous to treat waste sludge by means of rapid processes in which the sludge is utilized in producing solid products, such as biofuel. The basic method used in such processes consists of mixing organic binding agents with the waste sludge, forming grains, pellets or similar particles from the mixture by means of pressing or by using pressing as an intermediate phase of the process, and then drying them. However, there are several problems involved in such processing of waste sludge. First of all, it is difficult to efficiently extract water from the sludge. It is difficult because the water is bound to the cell structure of the sludge. Waste sludge is usually dried after compaction by means of centrifuging or a pressing method such as filter pressing. Even after drying by means of such a reasonably quick method that consumes a reasonable amount of energy, the water content of the waste sludge is typically in the magnitude of 70-80%. A second problem involves formation of a coherent mass that may be shaped without having to use such an abundance of coagulants or moisture-binding organic additives that it is not economically feasable to use the process extensively. A third problem involves drying the products formed from the material to make them stabile and coherent. This means that, for example, bacterial action must be stopped so it will not degrade the product.
Attempts have been made to treat waste sludge, for example, by adding milled peat to it and then pressing it. However, a pressing method has not been found that efficiently and economically extracts water from such a mixture and allows easily shaped and dried, coherent products to be made from the mixture. A number of proposals have been made regarding methods by which solid products suitable for use as fertilizers or topsoil additives may be produced from agricultural waste sludge, in particular, and suitable organic additives. However, such proposals, one of which is examined in more detail below, do not present an efficient solution to the above-mentioned problems. U.S. Pat. No. 5,389,114 presents a process for producing biofuel that includes several mechanical and chemical treatment phases and disinfection phases, of which the last is microwave radiation. The disadvantage of this method is its complexity and on the other hand, the fact that it is largely based on the use of chemicals.
Swiss Patent No. 683519 presents a solution in which an organic additive, such as straw or sawdust, is mixed into agricultural sludge and the mixture is strongly extruded to produce a homogenous mass and then heated and dried to produce a nearly dry product. The extrusion may be repeated, after which drying may also be repeated.
German Patent Application No. 4110950 presents a solution in which a topsoil additive is produced from underwater sludge by mixing it with particle-, fiber- or flake-like organic material with a large outer surface and low moisture content, thus eliminating the sludge structure, after which the resulting mixture may be further mixed and then granulated and dried. In light of the presented examples, it appears that the method is suitable for treating sludge with a composition that meets certain requirements and has a quite low water content.
In the method of both documents cited above, the process is based on simply mixing a sufficiently suitable organic additive with the sludge to produce a mass suitable for further treatment. In order for this to be economical on a large scale, the characteristics of the waste sludge must be especially suitable for the treatment. If a large quantity of additive is required, for example, noticeably more than the quantity of waste sludge itself, the economy of the method of treatment becomes poor, as it was already stated above.